In these years, recording media for recording high-definition and ultra wideband sources such as a DVD-Audio and a Super Audio CD have been widely spread on the market. In order to reproduce these sources, a speaker capable of reproducing ultrahigh frequencies up to about 100 kHz (what is called a tweeter or a supertweeter) has been demanded. With the price reduction of recording media such as the DVD-Audio and the Super Audio CD and of reproducing apparatuses thereof, an inexpensive speaker capable of reproducing up to ultrahigh frequencies has been demanded as a single unit component or an element for a compact stereo.
Japanese Laid-open Patent Application No. 2000-333295 describes a speaker in accordance with a first prior art, having: a cone-shaped diaphragm, an outer peripheral part of which is supported by a frame; and a monomorph type piezoelectric ceramic vibrator connected to a top part of the cone-shaped diaphragm (FIG. 5 of the Laid-open Patent Application).
The above-mentioned Laid-open Patent Application describes a speaker in accordance with a second prior art, having: a frame; a cone-shaped diaphragm, the outer peripheral part of which is fixed to the frame with an adhesive; a dome-shaped diaphragm having contact with an inner peripheral part of the cone-shaped diaphragm; and a piezoelectric element adhered to the outer peripheral part of the dome-shaped diaphragm (FIG. 6 of the Laid-open Patent Application).
The above-mentioned Laid-open Patent Application discloses a speaker for high frequency in accordance with a third prior art, which is improved in performance as compared to the first and second prior arts, having a structure wherein the diaphragm is attached to the piezoelectric ceramic vibrator (FIG. 1 of the Laid-open Patent Application).
A speaker for high frequency in accordance with the third prior art will be explained with reference to FIG. 14 to FIG. 16. FIG. 14 is a view showing a structure of the speaker for high frequency in accordance with the third prior art.
In FIG. 14, a numeral 21 designates a piezoelectric ceramic vibrator, a numeral 22 designates a frame, a numeral 23 designates a dome-shaped diaphragm, a numeral 24 designates an opening, and a numeral 25 designates a fixing member.
The piezoelectric ceramic vibrator 21 is a circular ring-shaped ceramic piezoelectric element, in which silver electrodes are provided on the both faces thereof and which are polarized in the through-thickness direction. In the piezoelectric ceramic vibrator 21, the inner peripheral part is fixed to the frame 22 via the fixing member 25 of an elastic body. The piezoelectric ceramic vibrator 21 expands and contracts in the radial direction and vibrates evenly throughout a circumference thereof. The dome-shaped diaphragm 23 of 20 mm diameter, which is formed of 35 μm thick polyetherimide films, is fixed to the outer peripheral part of the piezoelectric ceramic vibrator 21 with the adhesive. The dome-shaped diaphragm 23 converts the radial vibration of the piezoelectric ceramic vibrator 21 into the vertical vibration. The above-mentioned structure enables the speaker for high frequency in accordance with the third prior art to achieve wide radiation area, high sound pressure level, and fewer irregularities in the sound pressure frequency response than the case when the cone-shaped diaphragm or the like is used. FIG. 16 shows the sound pressure frequency response of the speaker for high frequency in accordance with the third prior art (wherein the frequency is shown on the horizontal axis and the sound pressure is shown on the vertical axis, and the same applies hereinafter). The speaker for high frequency in accordance with the third prior art has shown its efficient performance in reproducing a conventional source having a frequency band of 20 kHz or less.
In the speaker for high frequency in accordance with the third prior art, the circular ring-shaped piezoelectric ceramic vibrator 21 is fixed in the inner peripheral part, and the diaphragm 23 is attached to the outer peripheral part which is a counter pole thereof. The parts (a) to (c) of FIG. 15 show three vibration modes of the circular ring-shaped piezoelectric ceramic vibrator which is fixed in the inner peripheral part. The upper drawings of the parts (a) to (c) of FIG. 15 are plan views showing the vibrating piezoelectric ceramic vibrator 21. In FIG. 15, the part (a) shows a primary (fundamental frequency) mode, the part (b) shows a secondary node circle mode, and the part (c) shows a tertiary node circle mode. The hatched part shows the displacement in the opposite direction to the non-hatched part (i.e., the boundary between the hatched part and the non-hatched part is a node of the vibration).
The bottom drawings of the parts (a) to (c) of FIG. 15 show the state of displacement of the piezoelectric ceramic vibrator (wherein the amplitude of the vibration is shown on the axis of ordinate, and the piezoelectric ceramic vibrator vibrates in the radial direction actually). As shown in FIG. 15, the outer peripheral part of the piezoelectric ceramic vibrator 21, to which the dome-shaped diaphragm 23 is connected, becomes an antinode in all vibration modes. The vibration only in the outer peripheral part of the piezoelectric ceramic vibrator 21 is transmitted to the dome-shaped diaphragm 23, being likely to cause the speaker for high frequency in accordance with the third prior art to generate resonance in the structure. Hence, according to the structure in accordance with the third prior art, peak/dip of the sound pressure frequency response become very large. As shown in FIG. 16, the speaker for high frequency in accordance with the third prior art has a large peak in the vicinity of about 27 kHz in sound pressure frequency response thereof.
In the speaker for high frequency using a circular shaped piezoelectric ceramic vibrator without modification, since its impedance is very high, flat sound pressure frequency response will not be obtained, and, furthermore, sound pressure level will be low. The speaker in accordance with the third prior art obtained high sound pressure level by making the diaphragm area large. The diaphragm of the speaker in accordance with the third prior art was therefore allowed to become large in diameter. Generally, in a speaker, a larger diaphragm degrades the directional pattern.
The upper cut-off frequency of sources reproduced by the DVD Audio or the Super Audio CD is about 96 kHz. The speaker for high frequency in accordance with the third prior art has not been capable of reproducing efficiently such high-definition and ultra wideband sources in performance. As shown in FIG. 16, the speaker for high frequency in accordance with the third prior art has large peak/dip in the range exceeding 20 kHz and can obtain an efficient sound pressure only up to about 40 kHz.
The piezoelectric ceramic vibrator 21 used in the speaker for high frequency in accordance with the third prior art has a circular ring-shaped special form, resulting in very high cost.
The present invention is to solve the above-mentioned conventional problems, and an object thereof is to provide an inexpensive speaker for reproducing ultrahigh frequencies, having superior sound pressure frequency response wherein the peak/dip is small and the upper cut-off frequency exceeds 100 kHz, high sound pressure level, and excellent directional pattern.